The invention relates to a converter for generating a video signal from a computer graphics signal.
Television (TV) technology has evolved from the necessity of minimising the transmission bandwidth required to carry moving natural-world images. Current TV standards have comparatively low spatial resolution, a chrominance bandwidth which is much less than the luminance bandwidth, and a 2:1 field interlace.
Personal computer (PC) display technology is, like television, also based largely on CRTs (cathode ray tubes) but has different constraints and objectives. Transmission bandwidth is not relevant and the images usually contain fine detail of high contrast, such as text. Satisfactory display of high-contrast single-pixel details require much more bandwidth than is required for natural world images, and non-interlaced scanning is almost essential. (The terms "sequential" or "progressive" are sometimes used as synonyms for "non-interlaced".) The frame refresh rate has to be at least 60 Hz (often 70 to 75 Hz) to eliminate flicker effectively. The combination of medium to high spatial resolution, non-interlaced scanning, and high refresh rate, results in a higher horizontal frequency (line rate) than is used in television.
For many applications the incompatibility between TV and PC is no problem, but as powerful computers and better graphics standards have become commonplace, advances in hardware and software have made it possible to produce sophisticated presentations and animations. We have appreciated that it would be highly desirable if the PC graphics output could simply be treated as a source of video. It could then be displayed on a TV-standard monitor or by a video projector, recorded on videotape, or even used in broadcasting.
Although the normal graphics mode of a typical personal computer is incompatible with the baseband video standards, many existing graphics cards (circuit boards) can in principle be run in a TV mode (often NTSC only). This involves setting an appropriate resolution, say 640.times.480 active pixels in the 525 line frame, and operating 2:1 field interlace at the standard line rate of 15.734 kHz.
In practice, this TV mode is often difficult to access, requiring low-level programming or installation of optional components on the graphics card, and may not be supported by the applications software. Other practical difficulties are that the PC's own monitor is often a "Fixed Frequency" type and will not display output at TV rates, and the interlacing causes severe flicker if there is fine detail present in the image. Even if these problems can be circumvented, the RGB output of such boards has to be externally encoded into composite video to be usable with most video equipment. It is however a low cost solution if it is feasible.
Alternatively, a separate external unit can be used to change the PC's normal graphics mode output to a TV rate by converting the line rate and, in most cases, the frame rate. This requires the analogue RGB from the graphics card to be re-sampled and buffered in a frame store. Most graphics modes do not have a simple relation to NTSC, so the number of lines in the frame may have to be altered by vertical scaling. After rate conversion and scaling, encoding into baseband video is straightforward, and is usually included in the same unit. An example of a scan converter using a frame store is described in U.S. Pat. No. 4,924,315 to Yamashita.
The extra flexibility of this scan conversion approach provides compatibility with a wider range of graphics cards and operating modes, and confers a large degree of transparency to the application and the user. Vertical filtering can be provided to reduce the flicker effect. It is possible to continue to use the PC's monitor while video is being produced by the scan converter, because the input to the scan converter is at normal graphics rates. The disadvantage is the complexity, because it needs to store a complete video frame, and consequently the cost of the scan converter itself.
It should also be noted that non-interlaced to interlaced signal converters are of themselves known, as for example described in U.S. Pat. No. 4,200,887 to Van Den Avoort and U.S. Pat. No. 4,386,367 to Peterson et al.